1. Field of the Invention
The present invention relates to a planar array antenna for radar sensors, having a plurality of base antenna elements arranged in a common plane on a printed circuit board and having additional antenna elements which are arranged in an offset plane in each case above one of the base antenna elements. In particular, the present invention is concerned with an array antenna for radar sensors, which are used in motor vehicles in connection with driver assistance systems, for instance, for position finding of preceding vehicles in an automatic adaptive cruise control system. These radar sensors typically operate at a frequency of 24 GHz or 77 GHz.
2. Description of the Related Art
In these applications, planar antennas in microstrip technology have the advantage that they may be produced comparatively cost-effectively, and make a flat construction possible, and that no junctions between different line systems or line types are required. The base antenna elements may be formed on a printed circuit board in the same way as the other circuit components, simply by appropriate microstrip conductors. The desired directional pattern of the antenna is achieved in that the microwave power, which is supplied to the individual base antenna elements via a feed line, is subdivided, with the aid of power transformers, in such a way to the antenna elements that, by interference, a main lobe is obtained, and minor lobes are extensively suppressed. The base antenna elements, in this instance, should be developed in such a way that, within a certain bandwidth, a good impedance matching is achieved and reflections of the supplied microwave power are avoided. The bandwidth is a function of the thickness of the printed circuit board. At 77 GHz, using printed circuit boards having a thickness of 100 to 130 μm, a bandwidth of about 1 to 2 GHz is achievable.
However, taking into account unavoidable production tolerances, it would be desirable to attain a clearly greater bandwidth, such as one increased by a factor of 3. One known possibility for increasing the bandwidth is to assign an additional antenna element to each base antenna element, which is arranged in parallel to the associated base antenna element. In this context, the additional antenna elements have to be aligned correctly relative to the base antenna elements.
In one known layout, the base antenna elements, on the one hand, and the additional antenna elements, on the other hand, are situated on two congruent printed circuit boards, which are then layered congruently one over the other. However, producing such an antenna array is relatively costly. An additional disadvantage is that, on the printed circuit board, which carries the base antenna element, no additional HF circuit components of the radar sensor are able to be accommodated.
If, on the other hand, one obtains a smaller top view for the printed circuit board that carries the additional antenna elements, the alignment of the antenna elements relative to one another becomes even more difficult.